Formation of Water Layers on Graphene Surfaces

Although
graphitic materials were thought to be hydrophobic, recent
experimental results based on contact angle measurements show that
the hydrophobicity of graphitic surfaces stems from airborne contamination
of hydrocarbons. This leads us to question whether a pristine graphitic
surface is indeed hydrophobic. To investigate the water wettability
of graphitic surfaces, we use molecular dynamics simulations of water
molecules on the surface of a single graphene layer at room temperature.
The results indicate that a water droplet spreads over the entire
surface and that a double-layer structure of water molecules forms
on the surface, which means that wetting of graphitic surfaces is
possible, but only by two layers of water molecules. No further water
layers can cohere to the double-layer structure, but the formation
of three-dimensional clusters of liquid water is confirmed. The surface
of the double-layer structure acts as a hydrophobic surface. Such
peculiar behavior of water molecules can be reasonably explained by
the formation of hydrogen bonds: The hydrogen bonds of the interfacial
water molecules form between the first two layers and also within
each layer. This hydrogen-bond network is confined within the double
layer, which means that no “dangling hydrogen bonds”
appear on the surface of the double-layer structure. This formation
of hydrogen bonds stabilizes the double-layer structure and makes
its surface hydrophobic. Thus, the numerical simulations indicate
that a graphene surface is perfectly wettable on the atomic scale
and becomes hydrophobic once it is covered by this double layer of
water molecules.